Evaporative fuel treatment apparatuses are known in which the intake path of an internal combustion engine and a fuel tank are coupled through a purge passage. Evaporative fuel produced in the fuel tank is purged into the intake path. For example, U.S. Pat. No. 7,069,916 (i.e., JP-2005-90281A) discloses such a system.
The evaporative fuel in the fuel tank can be saturated, and the quantity of evaporative fuel produced in a fuel tank is large. Also, a concentration of purged evaporative fuel fluctuates less as compared with cases where the evaporative fuel in a fuel tank is absorbed into a canister and then purged from the canister into the intake path. The evaporative fuel is higher in combustion efficiency than fuel spray injected from a fuel injection valve. Therefore, an internal combustion engine can be more readily started by purging evaporative fuel during engine start from the fuel tank into the intake path.
In the technology disclosed in U.S. Pat. No. 7,069,916, a quantity of evaporative fuel purged from the fuel tank into the intake path is controlled by measuring the pressure and temperature of the interior of the fuel tank. A concentration of the evaporative fuel in the fuel tank is set according to these measured values.
However, this method has certain disadvantages. Specifically, vapor pressure differs according to the fuel property, i.e., the types of components and/or the ratio of fuel components in the fuel. Therefore, when fuel properties differ, the concentration of evaporative fuel in the fuel tank differs as well even though the pressure and temperature of the interior of the fuel tank are the same. Thus, when an evaporative fuel concentration is set according to the pressure and temperature of the interior of the fuel tank, the set evaporative fuel concentration and the actual evaporative fuel concentration can be different from each other depending on fuel property. As a result, a quantity of evaporative fuel purged into the intake path may be inaccurately controlled.